Gallium nitride high electron mobility transistors (HEMTs) are used in a number of applications due at least in part to their simultaneous high power and high frequency operation. The channel layer of these transistors is capable of supporting large electric fields, and thus, high voltages, at small drain-source spacing. The nearly lattice-matched barrier layer is capable of providing an effective barrier for carrier confinement, and thus, allows the transistor to deliver large current densities. For those HEMTs formed on silicon carbide substrates, desirable thermal and isolation properties can be achieved.
Typically, gallium nitride HEMTs are depletion-mode transistors (D-mode or “normally-on” transistors). Gallium nitride HEMTs of the enhancement-mode (E-mode or “normally-off” transistors) are more rare. Unlike gallium arsenide HEMTs, the channel charge of gallium nitride HEMTs generally cannot be controlled by uniform or delta doping in the barrier. Rather, the charge in the gallium nitride channel is controlled by spontaneous and piezoelectric polarizations of the channel-barrier interface. As illustrated in the conduction band diagram (solid black line) in FIG. 1, under a typical growth condition (gallium face), electrons in the gallium nitride channel 102 are typically attracted to the barrier layer 104, thereby forming a channel under zero bias conditions. The electron wave function (hashed black line) shows where the electrons will tend to accumulate.
Methods for producing E-mode gallium nitride HEMTs sometimes include treating the surface of the transistor with fluorine, thereby creating enough deep surface states in the gate to pin the Fermi level deep inside the band gap and deplete the gallium nitride channel of electrons. Modifying the surface of the transistor in such a way, however, may not be reproducible and can affect the device reliability.